i did not read all of the posts, too many for my time. but i have read that alka seltzer is a general anisthetic for fish and one tab will quickly put a fish to sleep.
there are a few bad diseases that are easily noticed and take fish quickly, but some fish only have a genetic immunity to survive for so long, regardless wich species.(SURVIVAL OF THE FITTEST). Saltwater fish in most cases have a 75% fatality rate. freshwater is far less in percentages, but most of the same fish that people lose would never have made it that far in the wild. sad but true!
As far as cutting fish up to kill them. And I say "kill them" rather than euthanisation because that is kind of harsh for your pet. If there is no obvious sign of major contaminate or disease fish can surprise you by pulling through.
If you are introducing something new, like flourish excel and your fish start acting weird; STOP USING IT!!!! and do a 25% water change every two days until you have about three changes. especially if you have the more sensative "blue rams".
Always remember: the less the better as far as chemical additives go. try not to go beyond the water dechlorination and water-aging chemicals.
With a carefully clean environment with recommended water and filter pad changes according to your set-up, you will have the best results. All the little bottom feeders and certain snails should be researched in order to achieve the ultimate ecosystem.